Depending on the flour type and scooping technique, a cup of flour will weigh a different number of ounces. Regular all-purpose flour weighs around 4.5 ounces per cup.
In the case of flour, volume is expressed in cups, whereas weight is expressed in ounces. This explains why a cup of very fine cake flour weighs roughly three quarters of an ounce less than a cup of heavier bread flour.
Moreover, the weight of flour varies according to the scooping technique. A cup of flour that has been packed weighs heavier than one that has been fluffed. Since weight measurements are more precise than volume measurements, some recipes list dry ingredients by weight in ounces.
Since fluid volume is likewise measured in ounces, a cup of liquid always contains exactly 8 fluid ounces.